We are studying molecular-biological, biochemical and cell-biological responses of human cells to environmental stress. Some of the work deals with definition of conditions required for cellular injury; others with characterization of mechanisms by which a specific agent causes cellular damage to DNA, the energy metabolism of a cell or its membranous structure. To have a source of human skin cells, researchers must obtain from volunteers small samples of skin by skin biopsy. The procedure selected will be determined by the type of analysis required. The studies are relevant to fundamental biological questions concerning the nature of differentiation, aging, photosensitization, wound healing and cancer.